Symbolic gestures are called “mudras.” Mudra derives from the word mud, which means to delight. Basically mudras are seals that lock in energy and awareness. They can be formed by the hands or the body itself. The shapes represent inner states. This is easy to verify for yourself. Fold your arms in front of you and notice your inner state. Now release the arms and open them wide, joining the tips of your forefingers to your thumbs. Notice the changed inner state. Yogis who employ mudras can sense the energetic differences of even minor changes.
There are at least one hundred and eight hand gestures.  When we add all the body seals, we find there are a lot of mudras available. Some of the commonest hand seals are the anjali (prayer) mudra and the jnana (wisdom) mudra.  The jnana mudra is sometimes mistaken for the chin (consciousness) mudra. Swami Satyananda Saraswati, who has written many of the excellent books published by the Bihar School of Yoga in India, explains that the jnana mudra is done with the palm facing down, while the chin mudra has the palm facing up. Since jnana means wisdom, when we perform the jnana mudra we are sealing in wisdom. When we perform the chin mudra we are sealing in awareness or consciousness. These mudras are formed by touching the tip of the forefinger to the thumb. The jnana mudra is just one of many therapeutic mudras, and is said to be good for curing insomnia, nervous tension, and a weak memory.
There are many books available on mudras. In Swami Satyananda Saraswati’s book Asana Pranayama Mudra Bandha he has provided a full description of the five kinds of mudras: hand mudras, head mudras, postural mudras, lock mudras (bandhas), and perineal mudras. The book gives many examples of each kind of mudra, including a description for making the whole body into one big postural mudra: the maha mudra. 
Mudras are easily practiced in Yin Yoga. While in most of the postures, we can hold our hands in mudra or clasp our feet. Once the mudra is formed, the practice is then to sense the energetic effect of the mudra. That is a complete meditation all on its own.
- — One hundred and eight is a magical number signifying wholeness. When you see a claim that there are one hundred and eight of something or other, you can be assured there are probably a lot more than that.
- — Pronounced ghee-yana, and sometimes spelled gyana or gian mudra.
- — As shown in the picture above, sit down with one leg forward, the other knee bent so that the foot is brought to the inner thigh, press the heel into the perineum, and clasp the extended foot with both hands.